/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
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/*
 *
 *
 *
 *
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;
import java.util.AbstractQueue;
import java.util.Arrays;
import java.util.Collection;
import java.util.Comparator;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.PriorityQueue;
import java.util.Queue;
import java.util.SortedSet;
import java.util.Spliterator;
import java.util.function.Consumer;

/**
 * An unbounded {@linkplain BlockingQueue blocking queue} that uses
 * the same ordering rules as class {@link PriorityQueue} and supplies
 * blocking retrieval operations.  While this queue is logically
 * unbounded, attempted additions may fail due to resource exhaustion
 * (causing {@code OutOfMemoryError}). This class does not permit
 * {@code null} elements.  A priority queue relying on {@linkplain
 * Comparable natural ordering} also does not permit insertion of
 * non-comparable objects (doing so results in
 * {@code ClassCastException}).
 *
 * <p>This class and its iterator implement all of the
 * <em>optional</em> methods of the {@link Collection} and {@link
 * Iterator} interfaces.  The Iterator provided in method {@link
 * #iterator()} is <em>not</em> guaranteed to traverse the elements of
 * the PriorityBlockingQueue in any particular order. If you need
 * ordered traversal, consider using
 * {@code Arrays.sort(pq.toArray())}.  Also, method {@code drainTo}
 * can be used to <em>remove</em> some or all elements in priority
 * order and place them in another collection.
 *
 * <p>Operations on this class make no guarantees about the ordering
 * of elements with equal priority. If you need to enforce an
 * ordering, you can define custom classes or comparators that use a
 * secondary key to break ties in primary priority values.  For
 * example, here is a class that applies first-in-first-out
 * tie-breaking to comparable elements. To use it, you would insert a
 * {@code new FIFOEntry(anEntry)} instead of a plain entry object.
 *
 * <pre> {@code
 * class FIFOEntry<E extends Comparable<? super E>>
 *     implements Comparable<FIFOEntry<E>> {
 *   static final AtomicLong seq = new AtomicLong(0);
 *   final long seqNum;
 *   final E entry;
 *   public FIFOEntry(E entry) {
 *     seqNum = seq.getAndIncrement();
 *     this.entry = entry;
 *   }
 *   public E getEntry() { return entry; }
 *   public int compareTo(FIFOEntry<E> other) {
 *     int res = entry.compareTo(other.entry);
 *     if (res == 0 && other.entry != this.entry)
 *       res = (seqNum < other.seqNum ? -1 : 1);
 *     return res;
 *   }
 * }}</pre>
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @param <E> the type of elements held in this collection
 * @author Doug Lea
 * @since 1.5
 */
@SuppressWarnings("unchecked")
public class PriorityBlockingQueue<E> extends AbstractQueue<E>
    implements BlockingQueue<E>, java.io.Serializable {

  private static final long serialVersionUID = 5595510919245408276L;

    /*
     * The implementation uses an array-based binary heap, with public
     * operations protected with a single lock. However, allocation
     * during resizing uses a simple spinlock (used only while not
     * holding main lock) in order to allow takes to operate
     * concurrently with allocation.  This avoids repeated
     * postponement of waiting consumers and consequent element
     * build-up. The need to back away from lock during allocation
     * makes it impossible to simply wrap delegated
     * java.util.PriorityQueue operations within a lock, as was done
     * in a previous version of this class. To maintain
     * interoperability, a plain PriorityQueue is still used during
     * serialization, which maintains compatibility at the expense of
     * transiently doubling overhead.
     */

  /**
   * Default array capacity.
   */
  private static final int DEFAULT_INITIAL_CAPACITY = 11;

  /**
   * The maximum size of array to allocate.
   * Some VMs reserve some header words in an array.
   * Attempts to allocate larger arrays may result in
   * OutOfMemoryError: Requested array size exceeds VM limit
   */
  private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

  /**
   * Priority queue represented as a balanced binary heap: the two
   * children of queue[n] are queue[2*n+1] and queue[2*(n+1)].  The
   * priority queue is ordered by comparator, or by the elements'
   * natural ordering, if comparator is null: For each node n in the
   * heap and each descendant d of n, n <= d.  The element with the
   * lowest value is in queue[0], assuming the queue is nonempty.
   */
  private transient Object[] queue;

  /**
   * The number of elements in the priority queue.
   */
  private transient int size;

  /**
   * The comparator, or null if priority queue uses elements'
   * natural ordering.
   */
  private transient Comparator<? super E> comparator;

  /**
   * Lock used for all public operations
   */
  private final ReentrantLock lock;

  /**
   * Condition for blocking when empty
   */
  private final Condition notEmpty;

  /**
   * Spinlock for allocation, acquired via CAS.
   */
  private transient volatile int allocationSpinLock;

  /**
   * A plain PriorityQueue used only for serialization,
   * to maintain compatibility with previous versions
   * of this class. Non-null only during serialization/deserialization.
   */
  private PriorityQueue<E> q;

  /**
   * Creates a {@code PriorityBlockingQueue} with the default
   * initial capacity (11) that orders its elements according to
   * their {@linkplain Comparable natural ordering}.
   */
  public PriorityBlockingQueue() {
    this(DEFAULT_INITIAL_CAPACITY, null);
  }

  /**
   * Creates a {@code PriorityBlockingQueue} with the specified
   * initial capacity that orders its elements according to their
   * {@linkplain Comparable natural ordering}.
   *
   * @param initialCapacity the initial capacity for this priority queue
   * @throws IllegalArgumentException if {@code initialCapacity} is less than 1
   */
  public PriorityBlockingQueue(int initialCapacity) {
    this(initialCapacity, null);
  }

  /**
   * Creates a {@code PriorityBlockingQueue} with the specified initial
   * capacity that orders its elements according to the specified
   * comparator.
   *
   * @param initialCapacity the initial capacity for this priority queue
   * @param comparator the comparator that will be used to order this priority queue.  If {@code
   * null}, the {@linkplain Comparable natural ordering} of the elements will be used.
   * @throws IllegalArgumentException if {@code initialCapacity} is less than 1
   */
  public PriorityBlockingQueue(int initialCapacity,
      Comparator<? super E> comparator) {
    if (initialCapacity < 1) {
      throw new IllegalArgumentException();
    }
    this.lock = new ReentrantLock();
    this.notEmpty = lock.newCondition();
    this.comparator = comparator;
    this.queue = new Object[initialCapacity];
  }

  /**
   * Creates a {@code PriorityBlockingQueue} containing the elements
   * in the specified collection.  If the specified collection is a
   * {@link SortedSet} or a {@link PriorityQueue}, this
   * priority queue will be ordered according to the same ordering.
   * Otherwise, this priority queue will be ordered according to the
   * {@linkplain Comparable natural ordering} of its elements.
   *
   * @param c the collection whose elements are to be placed into this priority queue
   * @throws ClassCastException if elements of the specified collection cannot be compared to one
   * another according to the priority queue's ordering
   * @throws NullPointerException if the specified collection or any of its elements are null
   */
  public PriorityBlockingQueue(Collection<? extends E> c) {
    this.lock = new ReentrantLock();
    this.notEmpty = lock.newCondition();
    boolean heapify = true; // true if not known to be in heap order
    boolean screen = true;  // true if must screen for nulls
    if (c instanceof SortedSet<?>) {
      SortedSet<? extends E> ss = (SortedSet<? extends E>) c;
      this.comparator = (Comparator<? super E>) ss.comparator();
      heapify = false;
    } else if (c instanceof PriorityBlockingQueue<?>) {
      PriorityBlockingQueue<? extends E> pq =
          (PriorityBlockingQueue<? extends E>) c;
      this.comparator = (Comparator<? super E>) pq.comparator();
      screen = false;
      if (pq.getClass() == PriorityBlockingQueue.class) // exact match
      {
        heapify = false;
      }
    }
    Object[] a = c.toArray();
    int n = a.length;
    // If c.toArray incorrectly doesn't return Object[], copy it.
    if (a.getClass() != Object[].class) {
      a = Arrays.copyOf(a, n, Object[].class);
    }
    if (screen && (n == 1 || this.comparator != null)) {
      for (int i = 0; i < n; ++i) {
        if (a[i] == null) {
          throw new NullPointerException();
        }
      }
    }
    this.queue = a;
    this.size = n;
    if (heapify) {
      heapify();
    }
  }

  /**
   * Tries to grow array to accommodate at least one more element
   * (but normally expand by about 50%), giving up (allowing retry)
   * on contention (which we expect to be rare). Call only while
   * holding lock.
   *
   * @param array the heap array
   * @param oldCap the length of the array
   */
  private void tryGrow(Object[] array, int oldCap) {
    lock.unlock(); // must release and then re-acquire main lock
    Object[] newArray = null;
    if (allocationSpinLock == 0 &&
        UNSAFE.compareAndSwapInt(this, allocationSpinLockOffset,
            0, 1)) {
      try {
        int newCap = oldCap + ((oldCap < 64) ?
            (oldCap + 2) : // grow faster if small
            (oldCap >> 1));
        if (newCap - MAX_ARRAY_SIZE > 0) {    // possible overflow
          int minCap = oldCap + 1;
          if (minCap < 0 || minCap > MAX_ARRAY_SIZE) {
            throw new OutOfMemoryError();
          }
          newCap = MAX_ARRAY_SIZE;
        }
        if (newCap > oldCap && queue == array) {
          newArray = new Object[newCap];
        }
      } finally {
        allocationSpinLock = 0;
      }
    }
    if (newArray == null) // back off if another thread is allocating
    {
      Thread.yield();
    }
    lock.lock();
    if (newArray != null && queue == array) {
      queue = newArray;
      System.arraycopy(array, 0, newArray, 0, oldCap);
    }
  }

  /**
   * Mechanics for poll().  Call only while holding lock.
   */
  private E dequeue() {
    int n = size - 1;
    if (n < 0) {
      return null;
    } else {
      Object[] array = queue;
      E result = (E) array[0];
      E x = (E) array[n];
      array[n] = null;
      Comparator<? super E> cmp = comparator;
      if (cmp == null) {
        siftDownComparable(0, x, array, n);
      } else {
        siftDownUsingComparator(0, x, array, n, cmp);
      }
      size = n;
      return result;
    }
  }

  /**
   * Inserts item x at position k, maintaining heap invariant by
   * promoting x up the tree until it is greater than or equal to
   * its parent, or is the root.
   *
   * To simplify and speed up coercions and comparisons. the
   * Comparable and Comparator versions are separated into different
   * methods that are otherwise identical. (Similarly for siftDown.)
   * These methods are static, with heap state as arguments, to
   * simplify use in light of possible comparator exceptions.
   *
   * @param k the position to fill
   * @param x the item to insert
   * @param array the heap array
   */
  private static <T> void siftUpComparable(int k, T x, Object[] array) {
    Comparable<? super T> key = (Comparable<? super T>) x;
    while (k > 0) {
      int parent = (k - 1) >>> 1;
      Object e = array[parent];
      if (key.compareTo((T) e) >= 0) {
        break;
      }
      array[k] = e;
      k = parent;
    }
    array[k] = key;
  }

  private static <T> void siftUpUsingComparator(int k, T x, Object[] array,
      Comparator<? super T> cmp) {
    while (k > 0) {
      int parent = (k - 1) >>> 1;
      Object e = array[parent];
      if (cmp.compare(x, (T) e) >= 0) {
        break;
      }
      array[k] = e;
      k = parent;
    }
    array[k] = x;
  }

  /**
   * Inserts item x at position k, maintaining heap invariant by
   * demoting x down the tree repeatedly until it is less than or
   * equal to its children or is a leaf.
   *
   * @param k the position to fill
   * @param x the item to insert
   * @param array the heap array
   * @param n heap size
   */
  private static <T> void siftDownComparable(int k, T x, Object[] array,
      int n) {
    if (n > 0) {
      Comparable<? super T> key = (Comparable<? super T>) x;
      int half = n >>> 1;           // loop while a non-leaf
      while (k < half) {
        int child = (k << 1) + 1; // assume left child is least
        Object c = array[child];
        int right = child + 1;
        if (right < n &&
            ((Comparable<? super T>) c).compareTo((T) array[right]) > 0) {
          c = array[child = right];
        }
        if (key.compareTo((T) c) <= 0) {
          break;
        }
        array[k] = c;
        k = child;
      }
      array[k] = key;
    }
  }

  private static <T> void siftDownUsingComparator(int k, T x, Object[] array,
      int n,
      Comparator<? super T> cmp) {
    if (n > 0) {
      int half = n >>> 1;
      while (k < half) {
        int child = (k << 1) + 1;
        Object c = array[child];
        int right = child + 1;
        if (right < n && cmp.compare((T) c, (T) array[right]) > 0) {
          c = array[child = right];
        }
        if (cmp.compare(x, (T) c) <= 0) {
          break;
        }
        array[k] = c;
        k = child;
      }
      array[k] = x;
    }
  }

  /**
   * Establishes the heap invariant (described above) in the entire tree,
   * assuming nothing about the order of the elements prior to the call.
   */
  private void heapify() {
    Object[] array = queue;
    int n = size;
    int half = (n >>> 1) - 1;
    Comparator<? super E> cmp = comparator;
    if (cmp == null) {
      for (int i = half; i >= 0; i--) {
        siftDownComparable(i, (E) array[i], array, n);
      }
    } else {
      for (int i = half; i >= 0; i--) {
        siftDownUsingComparator(i, (E) array[i], array, n, cmp);
      }
    }
  }

  /**
   * Inserts the specified element into this priority queue.
   *
   * @param e the element to add
   * @return {@code true} (as specified by {@link Collection#add})
   * @throws ClassCastException if the specified element cannot be compared with elements currently
   * in the priority queue according to the priority queue's ordering
   * @throws NullPointerException if the specified element is null
   */
  public boolean add(E e) {
    return offer(e);
  }

  /**
   * Inserts the specified element into this priority queue.
   * As the queue is unbounded, this method will never return {@code false}.
   *
   * @param e the element to add
   * @return {@code true} (as specified by {@link Queue#offer})
   * @throws ClassCastException if the specified element cannot be compared with elements currently
   * in the priority queue according to the priority queue's ordering
   * @throws NullPointerException if the specified element is null
   */
  public boolean offer(E e) {
    if (e == null) {
      throw new NullPointerException();
    }
    final ReentrantLock lock = this.lock;
    lock.lock();
    int n, cap;
    Object[] array;
    while ((n = size) >= (cap = (array = queue).length)) {
      tryGrow(array, cap);
    }
    try {
      Comparator<? super E> cmp = comparator;
      if (cmp == null) {
        siftUpComparable(n, e, array);
      } else {
        siftUpUsingComparator(n, e, array, cmp);
      }
      size = n + 1;
      notEmpty.signal();
    } finally {
      lock.unlock();
    }
    return true;
  }

  /**
   * Inserts the specified element into this priority queue.
   * As the queue is unbounded, this method will never block.
   *
   * @param e the element to add
   * @throws ClassCastException if the specified element cannot be compared with elements currently
   * in the priority queue according to the priority queue's ordering
   * @throws NullPointerException if the specified element is null
   */
  public void put(E e) {
    offer(e); // never need to block
  }

  /**
   * Inserts the specified element into this priority queue.
   * As the queue is unbounded, this method will never block or
   * return {@code false}.
   *
   * @param e the element to add
   * @param timeout This parameter is ignored as the method never blocks
   * @param unit This parameter is ignored as the method never blocks
   * @return {@code true} (as specified by {@link BlockingQueue#offer(Object, long, TimeUnit)
   * BlockingQueue.offer})
   * @throws ClassCastException if the specified element cannot be compared with elements currently
   * in the priority queue according to the priority queue's ordering
   * @throws NullPointerException if the specified element is null
   */
  public boolean offer(E e, long timeout, TimeUnit unit) {
    return offer(e); // never need to block
  }

  public E poll() {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
      return dequeue();
    } finally {
      lock.unlock();
    }
  }

  public E take() throws InterruptedException {
    final ReentrantLock lock = this.lock;
    lock.lockInterruptibly();
    E result;
    try {
      while ((result = dequeue()) == null) {
        notEmpty.await();
      }
    } finally {
      lock.unlock();
    }
    return result;
  }

  public E poll(long timeout, TimeUnit unit) throws InterruptedException {
    long nanos = unit.toNanos(timeout);
    final ReentrantLock lock = this.lock;
    lock.lockInterruptibly();
    E result;
    try {
      while ((result = dequeue()) == null && nanos > 0) {
        nanos = notEmpty.awaitNanos(nanos);
      }
    } finally {
      lock.unlock();
    }
    return result;
  }

  public E peek() {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
      return (size == 0) ? null : (E) queue[0];
    } finally {
      lock.unlock();
    }
  }

  /**
   * Returns the comparator used to order the elements in this queue,
   * or {@code null} if this queue uses the {@linkplain Comparable
   * natural ordering} of its elements.
   *
   * @return the comparator used to order the elements in this queue, or {@code null} if this queue
   * uses the natural ordering of its elements
   */
  public Comparator<? super E> comparator() {
    return comparator;
  }

  public int size() {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
      return size;
    } finally {
      lock.unlock();
    }
  }

  /**
   * Always returns {@code Integer.MAX_VALUE} because
   * a {@code PriorityBlockingQueue} is not capacity constrained.
   *
   * @return {@code Integer.MAX_VALUE} always
   */
  public int remainingCapacity() {
    return Integer.MAX_VALUE;
  }

  private int indexOf(Object o) {
    if (o != null) {
      Object[] array = queue;
      int n = size;
      for (int i = 0; i < n; i++) {
        if (o.equals(array[i])) {
          return i;
        }
      }
    }
    return -1;
  }

  /**
   * Removes the ith element from queue.
   */
  private void removeAt(int i) {
    Object[] array = queue;
    int n = size - 1;
    if (n == i) // removed last element
    {
      array[i] = null;
    } else {
      E moved = (E) array[n];
      array[n] = null;
      Comparator<? super E> cmp = comparator;
      if (cmp == null) {
        siftDownComparable(i, moved, array, n);
      } else {
        siftDownUsingComparator(i, moved, array, n, cmp);
      }
      if (array[i] == moved) {
        if (cmp == null) {
          siftUpComparable(i, moved, array);
        } else {
          siftUpUsingComparator(i, moved, array, cmp);
        }
      }
    }
    size = n;
  }

  /**
   * Removes a single instance of the specified element from this queue,
   * if it is present.  More formally, removes an element {@code e} such
   * that {@code o.equals(e)}, if this queue contains one or more such
   * elements.  Returns {@code true} if and only if this queue contained
   * the specified element (or equivalently, if this queue changed as a
   * result of the call).
   *
   * @param o element to be removed from this queue, if present
   * @return {@code true} if this queue changed as a result of the call
   */
  public boolean remove(Object o) {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
      int i = indexOf(o);
      if (i == -1) {
        return false;
      }
      removeAt(i);
      return true;
    } finally {
      lock.unlock();
    }
  }

  /**
   * Identity-based version for use in Itr.remove
   */
  void removeEQ(Object o) {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
      Object[] array = queue;
      for (int i = 0, n = size; i < n; i++) {
        if (o == array[i]) {
          removeAt(i);
          break;
        }
      }
    } finally {
      lock.unlock();
    }
  }

  /**
   * Returns {@code true} if this queue contains the specified element.
   * More formally, returns {@code true} if and only if this queue contains
   * at least one element {@code e} such that {@code o.equals(e)}.
   *
   * @param o object to be checked for containment in this queue
   * @return {@code true} if this queue contains the specified element
   */
  public boolean contains(Object o) {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
      return indexOf(o) != -1;
    } finally {
      lock.unlock();
    }
  }

  /**
   * Returns an array containing all of the elements in this queue.
   * The returned array elements are in no particular order.
   *
   * <p>The returned array will be "safe" in that no references to it are
   * maintained by this queue.  (In other words, this method must allocate
   * a new array).  The caller is thus free to modify the returned array.
   *
   * <p>This method acts as bridge between array-based and collection-based
   * APIs.
   *
   * @return an array containing all of the elements in this queue
   */
  public Object[] toArray() {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
      return Arrays.copyOf(queue, size);
    } finally {
      lock.unlock();
    }
  }

  public String toString() {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
      int n = size;
      if (n == 0) {
        return "[]";
      }
      StringBuilder sb = new StringBuilder();
      sb.append('[');
      for (int i = 0; i < n; ++i) {
        Object e = queue[i];
        sb.append(e == this ? "(this Collection)" : e);
        if (i != n - 1) {
          sb.append(',').append(' ');
        }
      }
      return sb.append(']').toString();
    } finally {
      lock.unlock();
    }
  }

  /**
   * @throws UnsupportedOperationException {@inheritDoc}
   * @throws ClassCastException {@inheritDoc}
   * @throws NullPointerException {@inheritDoc}
   * @throws IllegalArgumentException {@inheritDoc}
   */
  public int drainTo(Collection<? super E> c) {
    return drainTo(c, Integer.MAX_VALUE);
  }

  /**
   * @throws UnsupportedOperationException {@inheritDoc}
   * @throws ClassCastException {@inheritDoc}
   * @throws NullPointerException {@inheritDoc}
   * @throws IllegalArgumentException {@inheritDoc}
   */
  public int drainTo(Collection<? super E> c, int maxElements) {
    if (c == null) {
      throw new NullPointerException();
    }
    if (c == this) {
      throw new IllegalArgumentException();
    }
    if (maxElements <= 0) {
      return 0;
    }
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
      int n = Math.min(size, maxElements);
      for (int i = 0; i < n; i++) {
        c.add((E) queue[0]); // In this order, in case add() throws.
        dequeue();
      }
      return n;
    } finally {
      lock.unlock();
    }
  }

  /**
   * Atomically removes all of the elements from this queue.
   * The queue will be empty after this call returns.
   */
  public void clear() {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
      Object[] array = queue;
      int n = size;
      size = 0;
      for (int i = 0; i < n; i++) {
        array[i] = null;
      }
    } finally {
      lock.unlock();
    }
  }

  /**
   * Returns an array containing all of the elements in this queue; the
   * runtime type of the returned array is that of the specified array.
   * The returned array elements are in no particular order.
   * If the queue fits in the specified array, it is returned therein.
   * Otherwise, a new array is allocated with the runtime type of the
   * specified array and the size of this queue.
   *
   * <p>If this queue fits in the specified array with room to spare
   * (i.e., the array has more elements than this queue), the element in
   * the array immediately following the end of the queue is set to
   * {@code null}.
   *
   * <p>Like the {@link #toArray()} method, this method acts as bridge between
   * array-based and collection-based APIs.  Further, this method allows
   * precise control over the runtime type of the output array, and may,
   * under certain circumstances, be used to save allocation costs.
   *
   * <p>Suppose {@code x} is a queue known to contain only strings.
   * The following code can be used to dump the queue into a newly
   * allocated array of {@code String}:
   *
   * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
   *
   * Note that {@code toArray(new Object[0])} is identical in function to
   * {@code toArray()}.
   *
   * @param a the array into which the elements of the queue are to be stored, if it is big enough;
   * otherwise, a new array of the same runtime type is allocated for this purpose
   * @return an array containing all of the elements in this queue
   * @throws ArrayStoreException if the runtime type of the specified array is not a supertype of
   * the runtime type of every element in this queue
   * @throws NullPointerException if the specified array is null
   */
  public <T> T[] toArray(T[] a) {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
      int n = size;
      if (a.length < n)
      // Make a new array of a's runtime type, but my contents:
      {
        return (T[]) Arrays.copyOf(queue, size, a.getClass());
      }
      System.arraycopy(queue, 0, a, 0, n);
      if (a.length > n) {
        a[n] = null;
      }
      return a;
    } finally {
      lock.unlock();
    }
  }

  /**
   * Returns an iterator over the elements in this queue. The
   * iterator does not return the elements in any particular order.
   *
   * <p>The returned iterator is
   * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
   *
   * @return an iterator over the elements in this queue
   */
  public Iterator<E> iterator() {
    return new Itr(toArray());
  }

  /**
   * Snapshot iterator that works off copy of underlying q array.
   */
  final class Itr implements Iterator<E> {

    final Object[] array; // Array of all elements
    int cursor;           // index of next element to return
    int lastRet;          // index of last element, or -1 if no such

    Itr(Object[] array) {
      lastRet = -1;
      this.array = array;
    }

    public boolean hasNext() {
      return cursor < array.length;
    }

    public E next() {
      if (cursor >= array.length) {
        throw new NoSuchElementException();
      }
      lastRet = cursor;
      return (E) array[cursor++];
    }

    public void remove() {
      if (lastRet < 0) {
        throw new IllegalStateException();
      }
      removeEQ(array[lastRet]);
      lastRet = -1;
    }
  }

  /**
   * Saves this queue to a stream (that is, serializes it).
   *
   * For compatibility with previous version of this class, elements
   * are first copied to a java.util.PriorityQueue, which is then
   * serialized.
   *
   * @param s the stream
   * @throws java.io.IOException if an I/O error occurs
   */
  private void writeObject(java.io.ObjectOutputStream s)
      throws java.io.IOException {
    lock.lock();
    try {
      // avoid zero capacity argument
      q = new PriorityQueue<E>(Math.max(size, 1), comparator);
      q.addAll(this);
      s.defaultWriteObject();
    } finally {
      q = null;
      lock.unlock();
    }
  }

  /**
   * Reconstitutes this queue from a stream (that is, deserializes it).
   *
   * @param s the stream
   * @throws ClassNotFoundException if the class of a serialized object could not be found
   * @throws java.io.IOException if an I/O error occurs
   */
  private void readObject(java.io.ObjectInputStream s)
      throws java.io.IOException, ClassNotFoundException {
    try {
      s.defaultReadObject();
      this.queue = new Object[q.size()];
      comparator = q.comparator();
      addAll(q);
    } finally {
      q = null;
    }
  }

  // Similar to Collections.ArraySnapshotSpliterator but avoids
  // commitment to toArray until needed
  static final class PBQSpliterator<E> implements Spliterator<E> {

    final PriorityBlockingQueue<E> queue;
    Object[] array;
    int index;
    int fence;

    PBQSpliterator(PriorityBlockingQueue<E> queue, Object[] array,
        int index, int fence) {
      this.queue = queue;
      this.array = array;
      this.index = index;
      this.fence = fence;
    }

    final int getFence() {
      int hi;
      if ((hi = fence) < 0) {
        hi = fence = (array = queue.toArray()).length;
      }
      return hi;
    }

    public Spliterator<E> trySplit() {
      int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
      return (lo >= mid) ? null :
          new PBQSpliterator<E>(queue, array, lo, index = mid);
    }

    @SuppressWarnings("unchecked")
    public void forEachRemaining(Consumer<? super E> action) {
      Object[] a;
      int i, hi; // hoist accesses and checks from loop
      if (action == null) {
        throw new NullPointerException();
      }
      if ((a = array) == null) {
        fence = (a = queue.toArray()).length;
      }
      if ((hi = fence) <= a.length &&
          (i = index) >= 0 && i < (index = hi)) {
        do {
          action.accept((E) a[i]);
        } while (++i < hi);
      }
    }

    public boolean tryAdvance(Consumer<? super E> action) {
      if (action == null) {
        throw new NullPointerException();
      }
      if (getFence() > index && index >= 0) {
        @SuppressWarnings("unchecked") E e = (E) array[index++];
        action.accept(e);
        return true;
      }
      return false;
    }

    public long estimateSize() {
      return (long) (getFence() - index);
    }

    public int characteristics() {
      return Spliterator.NONNULL | Spliterator.SIZED | Spliterator.SUBSIZED;
    }
  }

  /**
   * Returns a {@link Spliterator} over the elements in this queue.
   *
   * <p>The returned spliterator is
   * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
   *
   * <p>The {@code Spliterator} reports {@link Spliterator#SIZED} and
   * {@link Spliterator#NONNULL}.
   *
   * @return a {@code Spliterator} over the elements in this queue
   * @implNote The {@code Spliterator} additionally reports {@link Spliterator#SUBSIZED}.
   * @since 1.8
   */
  public Spliterator<E> spliterator() {
    return new PBQSpliterator<E>(this, null, 0, -1);
  }

  // Unsafe mechanics
  private static final sun.misc.Unsafe UNSAFE;
  private static final long allocationSpinLockOffset;

  static {
    try {
      UNSAFE = sun.misc.Unsafe.getUnsafe();
      Class<?> k = PriorityBlockingQueue.class;
      allocationSpinLockOffset = UNSAFE.objectFieldOffset
          (k.getDeclaredField("allocationSpinLock"));
    } catch (Exception e) {
      throw new Error(e);
    }
  }
}
